Fix isPaidImmediately's exact-match regression, add unpaid Vorkasse instructions

isPaidImmediately() matched paymentMethodTitle with an exact !== check
against the literal "Überweisung" — the consuming shop renamed its
Vorkasse row to "Überweisung (Vorkasse)" the same day, which would have
silently made every unpaid Vorkasse invoice show "✓ Bereits beglichen".
Changed to startsWith("Überweisung") — same design, tolerant of a
suffix qualifier.

Also adds an explicit instruction for the unpaid case (previously the
absence of the paid confirmation was the only signal, no actual text):
pay to the listed bank details, order processed after payment received
(~1-2 business days).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "@einfach-produktiv/invoicing",
-  "version": "0.1.0",
+  "version": "0.2.2",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@einfach-produktiv/invoicing",
-      "version": "0.1.0",
+      "version": "0.2.2",
       "dependencies": {
         "@e-invoice-eu/core": "^3.1.1"
       },